1. Field of the Invention
The invention relates generally to a method of making optical fluoride crystal elements. More specifically, the invention relates to a method of making an optical fluoride crystal blank that has its optical axis oriented along a specific crystallographic direction.
2. Background Art
Optical lithography systems are used in the manufacture of integrated circuits to print circuit patterns on a silicon surface. The optical lithography systems contain an illumination system and a projection system. The illumination system is used to irradiate a mask having a circuit pattern thereon, and the projection system is used to focus the pattern on the mask onto a silicon surface coated with a photoresist, thereby transferring the pattern on the mask onto the silicon surface.
The smallest feature size that can be printed is described by the following expression:       Feature    ⁢                  ⁢    size    =                    k        1            ⁢      λ        NA  where k1 is a proportionality constant that depends on the photoresist, specific illumination characteristics, mask geometry, and manufacturing process, λ is the illumination wavelength, NA is the numerical aperture of the objective. In general, the shorter the illumination wavelength, the smaller the feature size can be.
Many of the optical lithography systems currently on the market use ultraviolet light having a wavelength of 248 nm to print features as small as 0.25 μm. To obtain circuit features smaller than 0.25 μm, optical lithography systems using wavelengths shorter than 248 nm are needed. Optical lithography systems using 193 nm wavelength have been developed. Optical lithography systems using 157 nm wavelength are under active development. The 157-nm system is expected to print features as small as 100 nm.
Generally speaking, commercial use and adoption of the shorter-wavelength systems in high volume production of integrated circuits has been slow. This slow progression can be partly attributed to the stringent demands placed on the optical materials used in the projection system. The optical materials are required to have high transmittance properties at the illumination wavelength. The optical materials are required to be resistant to laser damage. The optical materials are required to have low residual index inhomogeneity, anisotropy, and birefringence.
The current trend in the industry is to use single grained fluoride crystals, for example calcium fluoride, barium fluoride, magnesium fluoride, and strontium fluoride crystals, to transmit wavelengths shorter than 200 nm. However, making fluoride crystal optical elements that meet the stringent demands required by the 193-nm and 157-nm lithography systems has been challenging. In particular, it is difficult to grow fluoride crystals that meet the low birefringence required in these systems. For instance, the target birefringence for fluoride crystals used in the 157-nm lithography system is less than 1 nm/cm.
Two types of birefringence phenomena are observed in fluoride crystals: stress-induced birefringence and intrinsic birefringence. Stress-induced birefringence is a consequence of the crystal growth process and can be reduced by improvement in the growth process. Intrinsic birefringence, on the other hand, has nothing to do with stress and cannot be reduced by improvement in the growth process. See, for example, Burnett, John H. et al, “Intrinsic Birefringence in Crystalline Optical Materials: A New Concern for Lithography,” Future FAB International Issue 12.
Burnett et al., supra, show that intrinsic birefringence has specific orientations with respect to the crystallographic directions. There are 12 birefringent maxima in the equivalent [110] directions, 6 birefringent zeros along the equivalent [100] directions, and 8 birefringent zeros along the equivalent [111] directions. This means that the optical axis of the crystal can be aligned with a specific crystallographic direction where intrinsic birefringence is zero or small. Also, multiple crystals elements having their optical axes aligned with specific crystallographic directions can be coupled together such that the net birefringence of the crystals is reduced in comparison to using only a single crystal orientation with the same focusing power.
It is important to verify that the optical axis of the crystal does not deviate from the desired crystallographic direction by more than a specified value. For example, for a [111] oriented calcium fluoride crystal, the maximum angular deviation that can be tolerated is ±5°. This is because birefringence increases rapidly for angles away from the [111] direction. Therefore, the smaller the angular deviation of the optical axis of the crystal from the [111] direction, the better the performance of the crystal will be when used in the lithography system. In general, the maximum angular deviation that can be tolerated will depend on the desired orientation of the optical axis.